Spectrometer

This application is a U.S. non-provisional application claiming the benefit of French Patent Application No. 24 04039 filed on Apr. 18, 2024, the contents of which are incorporated herein by reference in their entirety.

The present invention relates to a spectrometer. The present invention also relates to an associated optical design method.

A spectrometer works on the following principle: a slit is placed at the focus of a collimator which images this slit ad infinitum. A disperser (prism or grating) spectrally separates the beam from the collimator, then an optical imager focuses these spectral beams onto a detector.

The difficulty lies in carrying out these various operations with sufficient image quality, but there are many optical solutions available. The compactness of the spectrometer is a major criterion: the aim is to be as compact as possible, particularly for airborne applications.

There are many different types of spectrometer, but the most compact optical solution to date is the Dyson spectrometer, an example configuration of which is shown in. As shown in, a Dyson spectrometer includes a slit F, a detector D, a single lens L and a grating R deposited directly on a power mirror. The single lens L is used in double-pass mode for the beam coming from the slit and those going to the detector. In recent years, Dyson's compactness has been further enhanced by the use of Freeform technology for the single lens and the grating.

However, creating the grating is complex, even more so if a freeform component is added to the grating substrate. In addition, integrating the detector close to the slit of the Dyson is mechanically complex. Finally, because of its configuration, a Dyson spectrometer is limited to a magnification of 1, which is restrictive for certain applications.

There is therefore a need for a spectrometer that is less complex to build and integrate mechanically, while remaining compact and offering good image quality.

To this end, the aim of the invention is a spectrometer including:

In other beneficial aspects of the invention, the spectrometer includes one or more of the following features, taken in isolation or in any technically possible combination:

The invention also relates to a method of optical design of a spectrometer as previously described, the method being computer-implemented using an optical design tool, the method including:

A spectrometeris illustrated in.

As shown in the figure, the spectrometerincludes a slit, a detector, a diffraction grating, a collimating lens, a deflecting mirrorand a focusing lens.

Preferably, the collimating lens, the diffraction grating, the deflecting mirrorand the focusing lensare the only optics of the spectrometer.

Moreover, the diffraction gratingis advantageously a reflection diffraction grating, as shown in.

The slitis suitable for receiving a light beam. The slitis, for example, less than or equal to 20 millimeters (mm) long, preferably less than or equal to 15 mm.

The light beam is, for example, in the visible range (e.g. 380 to 780 nm). Alternatively, the light beam may also extend into the ultraviolet range (100 to 380 nm) or infrared range (780 nm to 100 μm).

The detectoris able to detect a light beam arriving at the detector. In particular, the detectoris able to detect light spectra and measure light intensity as a function of wavelength.

The diffraction gratingis capable of diffracting an incident beam into a plurality of diffracted beams. The diffracted beams are spectrally separated.

Preferably, the diffraction gratingis spherical. For example, the diffraction gratingis engraved onto a spherical mirror.

Alternatively, the diffraction gratingis aspherical or made using freeform technology. Freeform optics have no axis or center of symmetry, allowing a greater number of degrees of freedom in optical design.

The collimating lensis capable of sending the light beam from the slitonto the diffraction gratingso as to obtain a plurality of diffracted beams.

Preferably, the collimating lensis a spherical lens.

The deflecting mirroris designed to reflect the plurality of diffracted beams.

The focusing lensis designed to receive the plurality of diffracted beams reflected by the deflecting mirrorand to focus the plurality of diffracted beams on the detector.

At least one optic, referred to as an optimized optic, from among the collimating lens, deflecting mirror, and focusing lens, so as to improve the image quality of the image generated by the detectorfrom the plurality of diffracted beams. Improving image quality means reducing aberrations (compared to a non-optimized configuration).

Preferably, the or each optimized optic is a Freeform optic.

Preferably, the spectrometerincludes two optimized optics.

Preferably, at least one optimized optic is the deflecting mirror.

Preferably, at least one optimized optic is the focusing lens.

Thus, in a preferred embodiment:

In this preferred mode, the slitand the detectorare each preferably located close to the center of curvature of the collimating lens, the diffraction gratingand the focusing lens.

In this design, the deflecting mirrormechanically separates the slitand detector, and most importantly, by adding a Freeform surface, corrects the limiting aberrations (mainly astigmatism).

In one example embodiment, the collimating lensand the focusing lenshave different powers so that the magnification of the spectrometeris different from 1.

In one example embodiment, the powers of both the collimatingand focusinglenses are chosen so that the magnification of the spectrometeris between 0.2 and 5.

One example of a method for designing the spectrometerwill now be described. Such an optical design method is computer-implemented using an optical design tool. The optical design tool is, for example, Zemax software.

The design method includes an operation of loading a Dyson spectrometer, such as that shown in, into the optical design tool. Such a Dyson spectrometer includes:

The design method includes an operation of splitting the double-pass lens L so as to obtain:

The design method includes the operation of adding a deflecting mirror. The addition operation may be carried out before the splitting operation, or the two operations may be carried out simultaneously.

The design method includes an operation of rearranging the collimating lens, focusing lens, and deflecting mirrorso as to spatially separate the slit F and the detector D and so that the deflecting mirroris able to reflect the plurality of diffracted beams towards the focusing lens.

The design method includes an operation of optimizing at least one optic, referred to as an optimized optic, from among the collimating lens, deflecting mirror, and focusing lens, so as to improve the image quality of the image generated by the detector D from the plurality of diffracted beams.

As explained above, the optimized optics are, for example, Freeform optics. The optimized optic(s) is/are preferably the deflecting mirrorand/or the focusing lens.

The design method includes an operation of rearranging the slit F, the detector D, the diffraction grating R, the collimating lens, the focusing lensand the deflecting mirrorto obtain a mechanically adjustable spectrometer.

By virtue of its configuration, the spectrometerdoes not necessarily require a freeform grating (the grating may be spherical or aspherical, for example), while remaining compact like a Dyson and offering good image quality. In particular, the image quality (particularly astigmatism) is corrected by the position of the slit F. The resulting trefoil is corrected by modifying the focusing lens(which becomes different from the collimating lens).

In particular, simulations have been carried out for the preferred embodiment in which the collimating lensis a spherical lens, the diffraction gratingis spherical, the deflecting mirroris Freeform and the focusing lensis freeform. These simulations show that imaging quality is improved by 60% compared with a current Dyson spectrometer.

Such a spectrometeralso makes it possible to separate the slit and the detector, which facilitates the mechanical integration of the different elements, and in particular that of the detector with its electronics.

Finally, an interesting variation may be obtained by separating the collimating and focusing lenses: the spectrometer's magnification may differ from 1. Indeed, the symmetry associated with the magnification of 1 and the construction of the Dyson with a common center of curvature mean that certain aberrations (spherical aberrations, coma, Petzval curvature) may be naturally eliminated. By breaking this symmetry and the magnification of, the solution becomes less favorable for correcting aberrations. However, the gain brought about by the freedom of the slit and the position of the focusing lensmeans that it is possible to depart from a magnification of 1 while still maintaining an attractive image quality. This would not have been possible with a traditional Dyson or Freeform grating Dyson solution.

The choice of modifying the magnification (within a range of x0.2-x5) may be very useful in the overall design of the telescope and spectrometer. Usually, the only way to achieve this magnification while remaining relatively compact is to use an Offner spectrometer, which is not as compact as a Dyson.

Such a spectrometer may be used for many hyperspectral applications. This type of spectrometer is particularly suitable for use on a satellite in “push broom” mode. In particular, because it is so compact, the spectrometermay be mounted on small satellites. Optical instruments that may image a scene in a plurality of spectral domains are known as hyperspectral instruments (for example, imaging a visible scene inspectral domains 4 nm wide). This may lead to a wide range of information being extracted, such as unmasking military targets, determining the chemical composition of soil, isolating vegetation, determining the chemical composition of oceans, or determining the amount of plastic pollution in oceans or rivers.

The skilled person will appreciate that the above-described embodiments and variants may be combined to form new embodiments, provided that they are technically compatible.